This invention relates to crosslinking isocyanates for polymeric adhesives. Crosslinking additives, preferably for solvent-containing adhesives based on natural or synthetic polymers, include a number of trifunctional or higher polyisocyanates, for example 4,4',4"-triisocyanatotriphenyl methane, isocyanatoaryl phosphoric acid or thiophosphoric acid derivatives according to DE-PS 1,311,001 (for example thiophosphoric acid tris-(p-isocyanatophenyl ester)), substituted diphenyl methane tetraisocyanates according to DE-PS 1,812,565 or NCO-terminated reaction products of 2,4-tolylene diisocyanate with trimethylol propane. Other crosslinking isocyanates based on 2,4-tolylene diisocyanates are isocyanate-functional trimers of the type described, for example, in DE-PS 2,641,380.
The use of the polyisocyanates mentioned as crosslinking additives increases, above all, the heat resistance of the adhesive bond. For the purpose of arresting the elastic recovery forces, an adhesive film of high instant strength is required for the bonding of substrates under tension, for example in the bonding of shoe soles to highly curved upper parts or in the lamination of highly structured supporting components with preheated, i.e. softened, plastic films by vacuum forming.
Although some of these polyisocyanates, for example 4,4',4"-triisocyanatotriphenyl methane or the triisocyanatomonoisocyanurates according to DE-PS 2,641,380, do give extremely good instant strength values, a further improvement in this property would afford considerable technical advantages by enabling the cycle times to be shortened.
The present invention relates to mixtures of trimers of 2,4-tolylene diisocyanate with triisocyanatotriphenyl methane (4,4',4"-triisocyanatotriphenyl methane) as crosslinking isocyanates for polymeric adhesives which, compared with the already very effective individual components, surprisingly show both a distinctly improved instant strength in the peel test and also a distinct improvement in bond strength as measured in the presence of heat.